1. Field of the Invention
The present invention relates to compositions that are useful as polishing compositions for chemical-mechanical polishing of semiconductors. More specifically, the polishing compositions of the present invention include an aqueous medium, an oxidizing agent, an inhibitor and a pH buffer.
2. Description of Related Art
Methods for chemical mechanical polishing and planarization of substrates used in the formation of semiconductors using abrasive slurries are shown in Sandhu et al U.S. Pat. Nos. 5,994,224 issued Nov. 30, 1999, Streinz et al 5,993,686 issued Nov. 30, 1999, Kaufman et al 6,039,891 issued Mar. 21, 2000, Sandhu et al 6,040,245 issued Mar. 21, 2000. Chemical mechanical polishing methods using non-abrasive containing polishing compositions are shown in Hudson U.S. Pat. No. 5,927,792 issued Oct. 26, 1999 and in WO 99/61540 published Dec. 2, 1999.
In chemical mechanical polishing of semiconductors comprising silica having circuits therein of aluminum, titanium or titanium nitride, a polishing composition is required that results in a very low level of defects and provides a higher level of polishing performance than can be achieved with prior art polishing compositions that contain abrasives or that do not contain abrasives but other combinations of polishing components. Dishing of the metallic circuit of the semiconductor has been a problem with conventional polishing composition particularly those containing abrasives. Dishing of the metallic circuit occurs when significantly more of the center portion of a metallic circuit of a semiconductor is removed than is removed from the sides resulting in a dip in the circuit that is below the level of the surface of the semiconductor. A polishing composition and method are required that has an acceptable removal rate of material, does not result in scratching the surface of the semiconductor or in dishing of the metallic circuit of a semiconductor and provides excellent planarization of the surface of the semiconductor that is being polished.
An aqueous polishing composition for chemical mechanical polishing of semiconductor devices comprising silica and circuits of aluminum, titanium or titanium nitride; wherein said aqueous composition comprises
an oxidizing agent such as an alkali metal chlorate or hydrogen peroxide,
an inhibitor of a polyalkyleneimine, and
a pH buffer such as ammonium phosphate or an alkali metal carbonate and optionally, a complexing agent, oxide suppressants and other inhibitors can be added.
A further aspect of this invention is a method for polishing a semiconductor device comprised of silica and circuits of aluminum, titanium or titanium nitride by applying the novel aqueous a polishing composition at an interface between a polishing pad and the semiconductor device.
The aqueous polishing composition of this invention is used to polish semiconductor devices of a silica wafer having circuits therein of aluminum, titanium or titanium nitride. The composition when used in a typical polishing process provides excellent planarization of the surface, does not scratch the surface and in particular does not dish the metallic circuit but polishes it evenly and at about the same level as the silica substrate. The composition can be used in combination with a variety of conventional polishing pads that are used on typical polishing machines.
The novel aqueous polishing composition contains about 0.0001-30%, based on the weight of the composition, of an oxidizing agent; about 0.0001-15% by weight, based on the weight of the composition, of an inhibitor; and 0.0001-5.0% by weight, based on the weight of the composition, of a pH buffer to provide the composition with a pH in the range of about 2-11. Optionally, a complexing agent, oxide suppressants and other inhibitors can be added to the composition.
Typical oxidizing agents that can be used are alkali metal chlorates such as potassium chlorate; and other oxidizing agents can be used such as ammonium chlorate, potassium iodate, ammonium perchlorate, potassium hyperchlorite, ammonium hyperchlorite, potassium chlorite, ammonium chlorite and the like. Hydrogen peroxide also is a useful oxidizing agent. The oxidizing agent oxidizes the surface of the metal circuit so that the metal is readily removed in the polishing process.
The inhibitor is a polyalkyleneimine having a weight average molecular weight of about 1,000-1,000,000. Preferably, polyethyleneimine is used. The inhibitor forms an inhibitor layer or a passivation layer on a metal substrate being polished such a aluminum and protects the substrate from dissolution during polishing and reducing dishing of the substrate. Depending on the pH of the polishing composition, the inhibitor layer can be a layer of aluminum oxide or a layer of an aluminum inhibitor polymeric film. At pH 4-9, the inhibitor layer is mainly aluminum oxide. When the pH is less than 2 or higher than 9, this inhibitor layer is an aluminum inhibitor polymeric film. The inhibitor layer is formed by the adsorption of inhibitor onto the aluminum surface through chemical bonding. When an organic inhibitor is used, such as polyethyleneimine, a polymeric inhibitor layer is formed and the thickness of the layer is built up by hydrogen bonding. The inhibitor layer can only be removed by mechanical abrasion and the removal rate of material during polishing is very low which results in a low level of dishing of the aluminum. Similar results occur when polishing titanium and titanium nitride since the surface of these compounds mainly is titanium oxide.
A pH buffer is used to keep the polishing composition at a pH of 2-11, preferably in the range of 4-9. Typical compounds that can be used are ammonium phosphate, ammonium hydrogen phosphate, potassium carbonate, ammonium tetraborate and potassium tetraborate.
About 0.0001-10% by weight, based on the weight of the composition, of an oxide suppressant can be added to the polishing composition. Typical oxide suppressants that can be used are organic bromides such as dodecyl trimethylammonium bromide. Under some circumstances, a polyalkyleneimines such as polyethyleneimine can function as an oxide suppressant. For example, when silicon dioxide is polished the polyalkyleneimine functions as an oxide suppressant.
Other inhibitors can be added to the polishing composition so long as the total amount of inhibitor including the polyalkyleneimine does not exceed 15% by weight of the polishing composition. Typical inhibitors that can be added to the polishing composition are inorganic compounds like phosphoric acid, phosphoric acid salts, carbonate salts, boric acid, boric acid salts, chromate salts, silicate salts, or organic compounds containing nitrogen, silicon, and or oxygen. Organic compounds also can be used. Typical organic compounds are benzoic acid and salts of benzoic acid. By using these inhibitors, the pH of the slurry can be extended to 2-11. Some of the above compositions like salts of acids can function as pH buffers in particular the salts of phosphoric and boric acids.
Optionally, about 0.0001-30% by weight, based on the weight of the composition, of a complexing agent can be added to the composition. Typical complexing agents that can be used are citric acid, ethylenetetraacetic acid, imnodiacetic acid, and nitrilotriacetic acid. The complexing agent dissolves the metal oxide formed by the oxidizing agent and it is readily removed in the dissolved state during the polishing process. A complexing agent is not required for composition having a high pH such as 10 and above since the hydroxy ions in the composition act as a complexing agent and no additional complexing agent is required.
In the formulation of the polishing composition, the constituents can be blended together in any order. A variety of polishing pads can be used with the polishing composition that are conventionally used in polishing semiconductors. One preferred polishing pad is a fixed abrasive pad SWR 159 manufactured by the 3 M Company. Also, a wide variety of polishing machines can be used. One typically useful machine is a Westech 372.
The following examples illustrate the invention. All parts and percentages are on a weight basis unless otherwise indicated.